The present invention relates to the preparation of substantially crystalline sodium aluminosilicates.
For a long period of time it has been common practice to use phosphates as builders in detergent compositions. Yet, lately it has been found that an excess of phosphates in waste waters can lead to an overfertilization of the natural waters. Even though detergent compositions are not the only source of this type of pollution, various efforts have been made to develop detergent compositions wherein the phosphate content is low, yet the overall activity is maintained. Yet, complexing agents, such as trisodium polyphosphates, have an essential importance for the course of the washing process and cannot easily be replaced. Yet, as far as their water softening function is considered, this can also be performed by other watersoftening materials.
Therefore it has been proposed to utilize naturally occuring and/or synthetic sodium aluminosilicates which are insoluble in water for watersoftening purposes. The following properties are required for such sodium aluminosilicates:
1. A small particle size, possibly a particle size of less than about 10.mu., preferably of from about 3 to about 6.mu., in order to ensure an optimum distribution throughout the washing liquid.
2. A very narrow particle size range, because too small particles are retained in a fabric during a laundering operation, whereas too large particles tend to sediment.
3. Poor adhesive capacity towards textile fibers. This is achieved by using crystalline substances; the adhesive power of amorphous substances is too strong.
4. A high calcium binding power.
5. A good wettability, in order to be able to rapidly develop its full activity.
Sodium aluminosilicates exhibit the advantage, that their calcium binding power which is dependent on the pH-value in the reaction medium, their concentration, and their particle size are not diminished under rising temperatures in the same manner as that of complexing agents. For certain ion exchangers, even an increase of the calcium binding power is observed under rising temperatures due to the increased diffusion rate and the reduction of the degree of hydration.
Amongst the zeolites, those of type A exhibit the highest calcium binding power. Therefore detergent manufactures are especially interested in the latter type of sodium aluminosilicate ion exchangers. These sodium aluminosilicates are usually prepared by reacting a sodium silicate solution with a sodium aluminate solution at elevated temperatures. The crystallization takes place during several hours at temperatures of about 100.degree. C.
According to a process disclosed in the German Auslegeschrift No. 1,038,015 synthetic molecular sieves are prepared by heating solutions containing sodium, silicate, and aluminate ions to a temperature of between 80.degree. and 100.degree. C., rapidly and thoroughly mixing them, and maintaining the mixture at this temperature for at least 5 hours. Yet, the resulting products are not satisfactory for the use as detergent additives. For example, they remain unwetted for a large period after being rinsed or sprinkled into water, and lump or float on the water surface (like sawdust). In view thereof the German Offenlegungsschrift No. 2510675 discloses a method for improving the wettability by thoroughly mixing alkali aluminosilicates with ortho phosphoric acid or alkali ortho phosphates, drying the mixture and subsequently grinding the dried product. Similarly, the use of pentaerythrite as is disclosed in the German Offenlegungsschrift No. 2510676, pentasodium triphosphate as is disclosed in the German Offenlegungsschrift No. 2510741, and tartaric acid as is disclosed in the German Offenlegungsschrift No. 2510742 have been proposed for improving the wettability of alkali aluminosilicates. For the same purpose, the addition of polycarboxylic acids is disclosed in the German Offenlegungsschrift No. 2345432.
The German Offenlegungsschriften Nos. 2333068, 2447021 and 2517218 disclose processes wherein synthesized mixtures are subjected to shearing forces in order to obtain a finely divided product.
From the disclosure of the German Auslegeschrift No. 1667620, it is known, that at an increased alkali concentration in the reaction mixture, the rates of crystallization and particle growth are increased, and that the latter becomes difficult to control unless appropriate precautions are taken. According to the teaching of the German Auslegeschrift, an increase of the alkali content leads to the formation of coarser grains, and chemically uniform precipitates can only be obtained if the silicate solution is introduced into the aluminate solution.